1. Field of the Invention
The present invention relates to a silicon nitridexe2x80x94tungsten carbide composite sintered material and process for preparing the same, and more particularly to a silicon nitridexe2x80x94tungsten carbide composite sintered material which is endowed with high strength, can attain reduced electrical resistance, exhibits reliable characteristics under wide ranges of firing conditions, and which avoids generation of tungsten silicide, which is a brittle substance. The present invention also relates to a glow plug employing the silicon nitridexe2x80x94tungsten carbide composite sintered material as a ceramic heating resistor.
2. Description of the Related Art
Conventionally, silicon nitridexe2x80x94tungsten carbide composite sintered material is used as a wear-resistant member such as a bearing ball or as a material for a heater of a glow plug, since the sintered material exhibits excellent wear resistance, and electrical resistance of the sintered material can be controlled easily. A typical approach to attain such intended characteristics of the sintered material is to regulate the amount of tungsten carbide to be incorporated into the sintered material. In order to meet the recent demand for enhancement of wear resistance and reduction of electrical resistance for attaining power savings, in some cases, the ratio of tungsten incorporated into the silicon nitridexe2x80x94tungsten carbide composite sintered material must be increased.
However, when the incorporation amount of tungsten carbide is increased in order to attain enhancement of wear resistance and reduction of electrical resistance with an aim toward power savings, sinterability of raw materials may be lowered significantly. As a result, an effective firing temperature range within which reliable characteristics are obtained is narrowed, and due to the non-uniformity of temperature within the firing furnace or changes in furnace temperature over the course of time caused by impairment of consumable parts of the furnace, reliable characteristics cannot be obtained. Meanwhile, conversion of tungsten carbide in sintered material into tungsten silicide, which is a brittle substance, through reaction between the tungsten carbide and silicon nitride causes deterioration of mechanical characteristics and electrical characteristics of the sintered material. This conversion tends to occur particularly in the vicinity of a firing jig formed from graphite. Thus, hitherto, there remains a need for a silicon nitridexe2x80x94tungsten carbide composite sintered material which exhibits excellent wear resistance, attains reduction of electrical resistance, realizes enhancement of sinterability, provides a wide effective firing temperature range within which reliable characteristics are obtained, and is endowed with high strength.
In view of the foregoing, an object of the present invention is to provide a silicon nitridexe2x80x94tungsten carbide composite sintered material which is endowed with high strength, can attain reduction of electrical resistance, exhibits reliable characteristics under wide ranges of firing conditions, and prevents generation of tungsten silicide, which is a brittle substance.
The present inventors have studied the relationship between components of a silicon nitridexe2x80x94tungsten carbide composite sintered material and characteristics of the sintered material, and have found that, when the amounts of rare earth elements as reduced to certain corresponding oxides thereof, the elements serving as sintering aid components, are regulated, and when the amount of excess oxygen as reduced to silicon dioxide is regulated, the sintered material exhibits high strength, electrical resistance can be reduced, an effective firing temperature range within which reliable characteristics are obtained can be widened, and conversion of tungsten carbide into tungsten silicide can be prevented. The present invention has been accomplished on the basis of this finding.
The present invention provides a silicon nitridexe2x80x94tungsten carbide composite sintered material comprising silicon nitride and tungsten carbide, characterized in that the total in amounts of an entirety of rare earth elements as reduced to certain corresponding oxides thereof, the elements being contained in the sintered material, and excess oxygen as reduced to silicon dioxide is 6-20 mass %; and the ratio, on a mol basis, of (the amounts of rare earth elements as reduced to the certain corresponding oxides thereof)/(the amounts of the rare earth elements as reduced to the corresponding oxides thereof+the amount of excess oxygen as reduced to silicon dioxide) is 0.3-0.7.
The present invention also provides a process for preparing the above-described silicon nitridexe2x80x94tungsten carbide composite sintered material, which comprises sintering a mixture of silicon nitride powder, an oxide of a rare earth element and silicon dioxide powder, the composite sintered material having a flexural strength of at least 800 MPa obtainable by sintering at an effective firing temperature which encompasses a range of at least 100xc2x0 C.
The present invention also provides a glow plug having a ceramic heater comprising the above-described silicon nitridexe2x80x94tungsten carbide composite sintered material.
In the silicon nitridexe2x80x94tungsten carbide composite sintered material of the present invention, a crystalline phase is present in an intergrain region of the sintered material. In the silicon nitridexe2x80x94tungsten carbide composite sintered material of the present invention, an effective firing temperature range within which a flexural strength of at least 800 MPa is obtained encompasses at least 100 degrees centigrade.
In the silicon nitridexe2x80x94tungsten carbide composite sintered material of the present invention, the aforementioned xe2x80x9crare earth elementxe2x80x9d is one or more elements selected from among, for example, Y, Sc, La, Ce, Pr, Nd, Gd, Tb, Dy, Er and Yb. The aforementioned xe2x80x9cthe amounts of rare earth elements as reduced to certain corresponding oxides thereofxe2x80x9d refers to the amounts of the rare earth elements as reduced to their oxides (i.e., RE2O3, RE: rare earth element), the elements being contained in the silicon nitridexe2x80x94tungsten carbide composite sintered material. In the silicon nitridexe2x80x94tungsten carbide composite sintered material of the present invention, the aforementioned xe2x80x9cexcess oxygenxe2x80x9d refers to oxygen contained in silicon nitride and oxygen in silicon dioxide that is added from the outside. The aforementioned xe2x80x9cthe amount of excess oxygen as reduced to silicon dioxidexe2x80x9d refers to the amount of the above-defined xe2x80x9cexcess oxygenxe2x80x9d as reduced to silicon dioxide. The aforementioned xe2x80x9cthe amounts of rare earth elements as reduced to certain corresponding oxides thereofxe2x80x9d is not particularly limited, so long as the total in amounts of an entirety of rare earth elements as reduced to certain corresponding oxides thereof and excess oxygen as reduced to silicon dioxide, and the ratio, on a mol basis, between the amounts of the rare earth elements and the amount of the excess oxygen satisfy the below-described conditions. The phrase xe2x80x9cas reduced toxe2x80x9d used herein implies xe2x80x9cif converted toxe2x80x9d or xe2x80x9cif present in the form ofxe2x80x9d, and does not necessarily imply any chemical reduction reaction.
In the silicon nitridexe2x80x94tungsten carbide composite sintered material of the present invention, the total of the amount of the rare earth element as reduced to the oxide thereof and the amount of the excess oxygen as reduced to silicon dioxide is usually 6-20 mass %, preferably 7-15 mass %, more preferably 7-11 mass %. When the total amount is less than 6 mass %, the amount of the sintering aid is insufficient for sintering of tungsten carbide and silicon nitride. Therefore, sinterability is lowered, and the fine structure of the sintered material tends to vary considerably with firing temperature. As a result, an effective firing temperature range within which reliable characteristics are obtained is narrowed, silicon nitride easily decomposes, and the resultant silicon component reacts with tungsten carbide to thereby generate a brittle substance; i.e., tungsten silicide, which is not preferable. In contrast, when the total amount exceeds 20 mass %, the electrical resistance of the sintered material increases, and the thermal shock resistance of the sintered material lowers, which is not preferable.
In the silicon nitridexe2x80x94tungsten carbide composite sintered material of the present invention, the ratio, on a mol basis, of (the amounts of rare earth elements as reduced to certain corresponding oxides thereof)/(the amounts of the rare earth elements as reduced to the corresponding oxides thereof+the amount of excess oxygen as reduced to silicon dioxide) is usually 0.3-0.7, preferably 0.3-0.5, more preferably 0.35-0.45. When the ratio is less than 0.3, the amount of the rare earth element is excessively small; i.e., the amount of the sintering aid is insufficient for sintering of tungsten carbide and silicon nitride, resulting in lowering of sinterability, which is not preferable. In contrast, when the ratio exceeds 0.7, the amounts of the rare earth elements are excessively large, and the total amount of the rare earth elements is not well balanced with the amount of the excess oxygen, resulting in insufficient sintering, which is not preferable.
Intergrain regions of the silicon nitridexe2x80x94tungsten carbide composite sintered material of the present invention may contain a crystalline phase in addition to an amorphous phase of, for example, rare earth elements, silicon, oxygen, or nitrogen. When a crystalline phase is present in the intergrain regions, softening of grain boundary glass at high temperatures is prevented, and mechanical characteristics of the sintered material at high temperature can be enhanced, which is preferable. Examples of the crystalline phase include RE2Si2O7 and RE2SiO5 (RE: rare earth element). The crystalline phase may contain one or more species of these.
Regarding the silicon nitridexe2x80x94tungsten carbide composite sintered material of the present invention, since sinterability of raw materials can be enhanced, an effective firing temperature range within which reliable characteristics are obtained can be widened, as compared with the case of conventional silicon nitridexe2x80x94tungsten carbide composite sintered material. Specifically, an effective firing temperature range within which a flexural strength of 800 MPa or more is obtained encompasses at least 100 degrees, preferably at least 140 degrees, more preferably at least 180 degrees. When raw materials of the sintered material have such an effective firing temperature range, regardless of non-uniformity of temperature within a firing furnace or time-course change in furnace temperature caused by impairment of consumable parts of the furnace, reliable characteristics of the sintered material can be obtained, which is preferable.
No particular limitation is imposed on the total amount of silicon nitride and tungsten carbide incorporated into the silicon nitridexe2x80x94tungsten carbide composite sintered material of the present invention. In the sintered material, the total amount is usually 80-95 mass %, preferably 85-95 mass %, more preferably 88-94 mass %. When the total amount falls within the above range, sinterability is not lowered, and the sintered material has low electrical resistance, which is preferable.
No particular limitation is imposed on the sintering method and sintering conditions for producing the silicon nitridexe2x80x94tungsten carbide composite sintered material of the present invention, so long as the sintered material can be produced. A sintering aid is not necessarily used, and sintering may be carried out at ambient pressure or at high pressure. The sintering temperature is usually 1,700-2,000xc2x0 C. In order to prevent decomposition of silicon nitride, sintering is usually carried out in a nonoxidizing gas atmosphere containing nitrogen.